This project continues to assess the role of the functional increase in opioid receptors observed in specific regions of the brains of CuZn superoxide dismutase mice. In order to assess if increases in mu receptors have any functional impact on the behavior of SOD-Tg mice, we have investigated the antinociceptive effects of opioids in these animals. Our results show that there were significant effects of genotype on antinociceptive dose-effects curves for each drug. The antinociceptive effect of morphine and of DAMGO were significantly influenced by the genotype of the subject. In addition, the dose of the transgene was a significant factor. Genotype did not influence U50488-induced analgesia in the transgenic SOD nor in the Non-Tg mice. These results suggest that the changes observed in Mu opioid receptors do have functional consequences. These studies also support the view that oxidative mechanisms might play an important role in nociception.